How do I use Hairnet properly?
I tried the addon, here it seems to work. 

If you set a seam on the "plane hair guide", like this
(note:I highlighted in red the "plane hair guide", in blue the "emitter")



then, in object mode, be sure to apply all transformations to the objects (location, rotation, scaling)

then you select those two in that order, and press this button



you should get something like this



which renders like this in blender render



I noticed though, that if you select the "emitter" (eg the head) first and after the "plane hair guide" (eg a custom shape) you get an error like "no seams were defined in plane", because it needs to be selected in the opposite way: first the "plane hair guide" (eg a custom shape), and then the  "emitter" (eg the head).

Otherwise, please describe how you set it up, or share the file on http://blend-exchange.giantcowfilms.com/, then paste the provided link editing your question: I'll take a look.